Interface method between remote radio unit and centralized base transceiver station

ABSTRACT

The present invention discloses a method for transmitting signal between a central channel processing Main Unit (MU) and one or more Remote Radio Units (RRUs) by using SDH/OTN in Centralized Base Transceiver Station system using remote radio head (RRH) technology. The method includes: dividing the communication interface between the MU and the RRUs into a user plane and a control plane, the user plane mainly for carrying I/Q sampling data concerning the user data, and the control plane mainly for carrying the data concerning control signaling; forming the I/Q sampling data concerning the user data carried by the user plane into an I/Q data frame via an I/Q data frame adaptation layer, then forming the I/Q data frame into a GFP frame via GFP and transmitting it on SDH/OTN; and carrying the control signaling of the control plane on UDP/IP and/or TCP/IP, and further carrying IP packet on PPP and forming it into a frame by HDLC, transmitting the HDLC frame including the control plane signaling on the SDH/OTN via the control character channel of the GFP frame. According to the present invention, the existing SDH/OTN transmission network is utilized directly, which further reduces the management and maintenance operation needed for signal transmission, as well as networking cost.

This application claims priority from PCT/CN2004/000800 filed Jul. 13,2004, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a Base Transceiver Station technologyused in mobile communication system, and more particularly, to aninterface method between Remote Radio Unit and Centralized Radio BaseStation in Centralized Base Transceiver Station system using RemoteRadio Head (RRH) technology.

BACKGROUND OF THE INVENTION 1. Centralized Base Transceiver Station(CBTS) and Wireless Signal Transmission

As illustrated in FIG. 1A, in mobile communication systems, wirelessaccess network is typically composed of Base Transceiver Stations (BTSs)and Base Station Controllers (BSCs) or Radio Network Controllers (RNCs)for controlling a plurality of BTSs, wherein the BSC is mainly composedof a base band processing subsystem, a radio frequency (RF) subsystem,and an antenna etc, which is responsible for transmitting, receiving,and processing wireless signal, a BTS can cover various cells by meansof a plurality of antennas, as illustrated in FIG. 1B.

In mobile communication systems, there are wireless network coverageproblems that are more difficult to solve with conventional BTStechnologies, such as, indoor coverage of high-rise buildings, coveragehole, or the coverage of shadow zone, RRH technology is a more effectivesolution being proposed to solve the above problems. In the BTS systemusing RRH technology, the primary radio frequency units and antennas areinstalled in regions that are required to provide coverage, and areconnected to other units in the BTS via broadband transmission lines.

This technology can be further developed to a CBTS technology that usesRRH technology. Compared with the conventional BTS, the CBTS using RRHtechnology has many advantages: the centralized structure allows to useseveral Micro-Cells to replace a Macro-Cell based on the conventionalBTS, therefore it can be adapted to various wireless environment better,and enhance wireless performances such as system capacity and coverageetc; the centralized structure enables the replacement of soft handoffin the conventional BTS by softer handoff, therefore obtains additionalprocessing gain; the centralized structure also enables valuable baseband signal processing resources to become a resource pool shared byseveral cells, therefore has the advantage of Statistic Multiplex, andalso decreases system cost. The following patents disclose someimplementation details about the CBTS using RRH technology, they are:U.S. Pat. No. 5,657,374, filed on Mar. 23, 1995, “Cellular system withcentralized base stations and distributed antenna units”, and U.S. Pat.No. 6,324,391, filed on Jun. 28, 1999, “Cellular communication withcentralized control and signal processing”, which are herebyincorporated by reference.

As illustrated in FIG. 2, the CBTS system 200 using RRH technology iscomposed of a central channel processing main unit (MU) (or a centralchannel processing subsystem) 201 and a plurality of Remote Radio Units(RRUs) 2041, 2042, . . . , 204M installed integrately. MU and RRUs areconnected to each other via broadband transmission links or network.BSC/RNC interface unit is responsible for performing user plane andsignaling plane processing of the interface between BTS and BSC/RNC. Thecentral channel processing subsystem 201 is mainly composed of a channelprocessing resource pool 202 and a signal route distribution unit 203,etc, wherein the channel processing resource pool 202 is formed bystacking a plurality of channel processing units 2021, 2022, . . . ,202N together, and is used to perform base band signal processing, etc.The signal route distribution unit 203 dynamically distributes thechannel processing resources in accordance with different cell traffics,to achieve efficient share of a plurality of cell processing resources.As illustrated in FIG. 2, besides inside the MU, the signal routedistribution unit 203 can be constructed as separate equipment outsidethe MU. The RRUs 2041, 2042, . . . , 204M are mainly composed of radiofrequency power amplifiers in transmission channel, low noise amplifiersin receiving channel, and antennas, etc (not shown entirely). Typicallythe link between the central channel processing subsystem 201 and theRemote Radio Units (RRUs) 2041, 2042, . . . , 204M can use transmissionmedia such as optical fiber, copper cable, microwave, etc; the signalstransmitted may be either sampled digital signals or modulated analogsignals; the signals may be either baseband signals or intermediatefrequency signals or radio frequency signals.

In the two BTS systems using RRH technology discussed above, the keyproblem to be solved is the wireless signal transmission between the RRUand the MU. Typically, analog intermediate frequency or analog radiofrequency signal transmission scheme is adopted, although it is easierto adopt analog signal transmission scheme, there will be disturbingcomponents, for example noise, etc, in analog lines, and the modulationof the signal transmitted will introduce nonlinear distortion. Inaddition, the analog transmission may decrease the utilization oftransmission line, and hamper the implementation of large capacitymultiplex technology. Therefore, it is difficult to adopt the analogtransmission scheme in large scale networking.

To solve the problems, the scheme of digital signal transmission isproposed in the following two patents: Chinese patent applicationCN1464666, filed on Jun. 11, 2002, entitled “A soft BTS system based onremote fiber and its synchronization method”, and another Chinese patentapplication CN1471331, filed on Jul. 2, 2003 (claimed priority of Jul.2, 2002), entitled “The BTS system in mobile communication”. The schemeof digital base band signal transmission is generally used to decreasethe requirement for transmission bandwidth as much as possible. Whereinpatent application CN1464666 disclosed only the simple method of usingthe optical fiber to transmit digital I/Q (In-phase/Quadrature) baseband signal between the RRU and the main BTS, that is, the digital I/Qbase band signal is converted to serial data stream by means of parallelto serial conversion at the transmitting end, and then transmitted tothe receiving end by an optical transmitter, and restored to the digitalI/Q base band signal by means of serial to parallel conversion afterreceived by the optical receiver at the receiving end. Patentapplication CN1471331 proposed a transmission technology of usingEthernet technology in physical layer, the technology uses continuousbit stream format specially defined instead of Ethernet MAC (MediaAccess Control) frame. At present, a corporation organization named CPRI(Common Public Radio Interface) is also engaged in the standardizationof the digital base band transmission between the RRU and the main BTS,and its technology specification can be obtained from the websitehttp://www.cpri.info/spec.html. This technology specification adopts atechnology similar to that adopted in patent CN1471331, that is,physical interface uses 1000 MB or 10 GB Ethernet standard, upper layeruses a continuous bit stream format user-defined, but CPRI only supportspoint to point link. Since the existing technology described aboveadopts specific protocol specification with regard to transmission layertechnology, not mature transmission technology, many potentialtechnology problems wait to be verified by practical systems, and theperiods of technology development and product development are longer,the cost of network construction is higher, which makes itdisadvantageous to be used in large scale networking.

2. Generic Framing Procedure (GFP)

Generic Framing Procedure (GFP) is recommended by ITU-T and ANSIjointly, and is used to adapt the data stream of block code or packettypes to continuous byte synchronization transmission channel, typicallyfor example the new technologies as SDH (Synchronous Digital Hierarchy)and OTN (Optical Transmission Network), the detailed technologyspecification thereof may refer to ITU-T G.7041 or ANSIT1X1.5/2000-024R3, which are hereby incorporated by reference. GFP canbe classified into frame mapping GFP (GFP-F) that supports PDU (ProtocolData Unit) and transparent GFP (GFP-T) that supports block code, whereinthe GFP-F can be used in the adaptation of protocol packet as IP/PPP(Internet Protocol/Point to Point Protocol), MPLS (Multi-Protocol LabelSwitching), and Ethernet MAC (Media Access Control), etc, and the GFP-Tcan be used to directly adapt block code character stream in 1000 MBEthernet line, etc, accordingly, some application requirements for verylittle time delay can be satisfied, but the utilization of the GFP-Ttransfer bandwidth is lower than that of GFP-F transfer bandwidth.

In FIG. 3, a frame structure of GFP-T type is illustrated schematically.The GFP-T frame is composed of a core header and a payload part. And thepayload part includes a payload header, payload and a selectable payloadFCS (Frame Check Sequence). The core header includes a PL1 fieldindicating the payload length and a core header error control field(cHEC), wherein the cHEC is functioned as GFP frame delineation similarto ATM (Asynchronous Transfer Mode) Cell delineation, as well asprovides error protection for the core header. The payload headerindicates payload types and provides error protection by tHEC, whereinPayload Type Identifier (PTI) indicates that the GFP-T frame carriesuser data when it is “000”, and indicates that the GFP-T frame carriesclient management information when it is “100”, payload FCS indicator(PFI) indicates if there is a payload FCS, User Payload Identifier (UPI)and the PTI together indicate the types of user data or clientmanagement information in the payload. In addition, the Extension HeaderIdentifier (EXI) indicates the presence of a selectable extension headerand its type. At present, a typical use of the extension header isproviding channel identifier (CID), therefore supporting the multiplexof multiple separate client signals. As illustrated in FIG. 3, thepayload in the GFP-T frame is super block with fixed length which isformed by 64B/65B code block according to certain sequence. Since thedirect adaptation of the transparent GFP now uses block code characterstream of an 8B/10B line code, 64B/65B code block includes user datacharacter and control character, so a flag bit is used to indicate ifthere is a control character in the 64B/65B code block. Wherein the high4 bits of the control character are used as the following controlcharacter indication and the position indication of the control code inthe original 8B/10B code stream, and the low 4 bits are used to transmitthe control code itself.

To ensure the transmission of the client signal, the bandwidth of thetransmission channel, for example SDH/OTN, etc, should be a little widerthan the bandwidth required by the GFP frame. Since the size of eachsuper block in the GFP-T frame is 536 bits, the bit length of the GFP-Tframe, L can be denoted as:L=L _(overhead)+536·N  (1)

where N is the number of the super blocks in the GFP-T frame,L_(overhead) is the overhead length of the core header, the payloadheader and the selectable payload FCS, etc, in the GFP-T frame. If theoriginal client signal rate is B_(C) bit/s, and the transmission channelbandwidth of SDH/OTN, etc, is B_(T) bit/s, with consideration that eachsuper block can carry a client signal stream of 512 bits, the number ofthe super blocks in the GFP-T, N should satisfy the followingexpression:

$\begin{matrix}{\frac{L}{B_{T}} < \frac{512 \cdot N}{B_{C}}} & (2)\end{matrix}$

So the minimum number of the super blocks required by GFP-T to satisfythe above condition, N is:

$\begin{matrix}{N_{\min} = \left\lceil \frac{B_{C}L_{overhead}}{{512B_{T}} - {536B_{C}}} \right\rceil} & (3)\end{matrix}$where symbol ┌x┐ indicates the minimum integer larger than or equal tox.

SUMMARY OF THE INVENTION

To solve the problems with the wireless signal transmission between theRRU and the main BTS in the existing technology, it is therefore anobject of the present invention to provide an interface method betweenRRU and MU in CBTS system using RRH technology. The invention proposed amethod for transmitting digital wireless signal between RRU and MU basedon GFP technology. The method directly uses mature Synchronous DigitalHierarchy (SDH)/Optical Transmission Network (OTN) Transmission networktechnology to realize digital wireless signal transmission between RRUand main BTS without the need for specific transmission technology,therefore further reduces the management and maintenance operationneeded for signal transmission as well as networking cost.

According to one aspect of the present invention, it is proposed amethod for transmitting signal between a central channel processing MainUnit (MU) and one or more Remote Radio Units (RRUs) in Centralized BaseTransceiver Station system using remote radio head (RRH) technology.Wherein the transmission channel between the MU and the one or more RRUsuses Synchronous Digital Hierarchy (SDH)/Optical Transmission Network(OTN), the method comprising: dividing the communication interfacebetween the MU and the RRUs (RRUs) into a user plane and a controlplane, the user plane mainly for carrying I/Q sampling data concerningthe user data, and the control plane mainly for carrying the dataconcerning the control signaling; forming the I/Q sampling dataconcerning the user data carried by the user plane into an I/Q dataframe via an I/Q data frame adaptation layer, then forming the I/Q dataframe into a GFP frame via Generic Framing Procedure (GFP) andtransmitting it on SDH/OTN; and carrying the control signaling of thecontrol plane on User Data Protocol (UDP)/Internet Protocol (IP) and/orTransmission Control Protocol (TCP)/Internet Protocol (IP), and furthercarrying IP packet on PPP and forming it into a frame by High-Level DataLink Control (HDLC), transmitting the HDLC frame including the controlplane signaling on the SDH/OTN via GFP control character channel.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood when reading the preferredembodiments of the invention in conjunction with accompanying drawings,therefore the benefits, features, and advantageous effects of thepresent invention will become more evident, wherein:

FIG. 1A schematically shows the structure of a wireless access networkin a conventional mobile communication system;

FIG. 1B schematically shows the basic structure of a BTS system in theconventional mobile communication system;

FIG. 2 schematically shows the structure of a CBTS system using RRHtechnology;

FIG. 3 schematically shows a GFP-T frame structure for block codecharacter stream according to GFP;

FIG. 4 schematically shows the structure of the interface between a RRUand a central channel processing main unit according to a preferredembodiment of the present invention;

FIG. 5 is the GFP frame structure of the interface mode between a RRUand a central channel processing main unit according to a preferredembodiment of the present invention;

FIG. 6 is the I/Q data frame structure of the interface mode shown inFIG. 5;

FIG. 7 is the I/Q data frame structure of the interface mode between aRRU and a central channel processing main unit according to anotherpreferred embodiment of the present invention;

FIG. 8A, 8B schematically shows the structures of the transmitting endand the receiving end respectively, when using the control character inGFP-T client data frame to transmit control plane signaling frame data,in the interface mode between a RRU and a central channel processingmain unit according to FIG. 4;

FIG. 9 schematically shows the definition of control character accordingto the present invention;

FIG. 10 schematically shows the illustration for wireless framessynchronization and measurement of round trip transmission time delayaccording to the present invention;

FIG. 11A, 11B schematically shows the illustration of time delayadjustment based on RTT measurement according to the present invention;and

FIG. 12 schematically shows clock recovery and RRU timing according tothe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objectives, benefits, and advantageous effects of the presentinvention, will be more clearly understood from the following detaileddescription of the invention taken in conjunction with the accompanyingdrawings.

1. Wireless Signal Transmission and RRU-MU Interface Protocol

FIG. 4 shows the structure of the RRU-MU interface protocol according tothe present invention. The interface is composed of user plane andcontrol plane. Wherein the user plane mainly carries the I/Q samplingdata concerning the user data. The I/Q sampling data are firstly formedinto I/Q data frame via an I/Q data frame adaptation layer, and thentransmitted on SDH/OTN via GFP-T. The control signaling of the controlplane is carried on UDP (User Data Protocol)/IP and/or TCP (TransmissionControl Protocol)/IP, and IP packet is carried on PPP and formed into aframe by HDLC (High-Level Data Link Control). At last the HDLC frameincluding the control plane signaling is transmitted on SDH/OTN viaGFP-T control character channel.

A RRU can typically support one or more carrier frequencies, so theRRU-MU interface protocol should support the transmission of a pluralityof carrier frequency wireless signals. In addition, in practicalwireless BTS system, multi-antenna technology is adopted more frequentlyto achieve enhanced wireless performance. The multi-antenna technologyincludes technologies such as transmitting diversity, receive diversity,Multi-antenna transmitting/receive (MIMO) and Smart Antenna or ArrayAntenna, etc. In the wireless BTS system adopting multi-antennatechnology, there is strict time and phrase relation among respectiveantenna signals, so the transmission time delays of respective antennasignals are required to be the same during transmission. Therefore theRRU-MU interface protocol should support the transmission ofmulti-antenna signals corresponding to the same carrier frequency, andensure that the transmission time delays of respective antenna signalsare the same during transmission.

Therefore, according to the present invention, FIG. 5 shows a GFP framestructure of a preferred RRU-MU interface mode. In this interface mode,the wireless signals corresponding to M (M≧1) carrier frequencies of aRRU are transmitted by different super blocks using time divisionmultiplex. In the GFP-T frame adopted in the interface mode shown inFIG. 5, core header and payload header which are 8 bytes in all complywith the GFP standard of ITU-T/ANSI without using extension header,payload FCS is optional, in GFP-T frame payload, the initial M superblocks (M×67 bytes) correspond to M different carrier frequenciesrespectively, and this structure is repeated p (P≧1) times sequentially,so the total number N of the super blocks in a GFP-T frame is pM. Thisscheme is practically M carrier frequencies time division multiplexingthe transmission bandwidth of GFP-T, and the transmission of respectivecarrier frequency wireless signals are independent with each other.

FIG. 6 shows the I/Q data frame format of such interface mode. Firstly,the I/Q baseband signals of carrier frequency #m (m=1, 2, . . . M) fromrespective antennas are sequentially arranged at the same sampling time,wherein the sequence of the sampling values of the I/Q baseband signalsfrom respective antennas is the same with the spatial location sequenceof antenna array or antenna group, the sampling values of the I/Qbaseband signals from the same antenna are sequentially arrangedaccording to quadrature component sampling values and In-phase componentsampling values. Therefore, if the number of the antennas is D, samplingbit width is W (typically is 4˜20), then at a certain sampling time, theI/Q data length of the carrier frequency is 2WD bits. AN I/Q data frameis composed of L I/Q data corresponding to the carrier frequency,wherein L I/Q data are sequentially arranged according to sampling timeincrement, so the total bit length is 2WDL. Since in GFP-T framing,alignment method is based on byte, the bit length of an I/Q data frameshould be multiple of 8, that is, the length should be WDL/4 bytes,meanwhile the length of the I/Q data frame should be as short aspossible, so as to reduce the inherent time delay induced during framingprocess. Since the sampling bit width and the number of the antennas arefixed, L is the minimal value which makes WDL/4 to be integral. Forexample, the antenna number D of a certain RRU is 2, sampling bit widthW is 11, then L=2, and the length of the I/Q data frame is 11 bytes.

FIG. 7 shows the I/Q data frame format of another preferred RRU-MUinterface mode. In this interface mode, the wireless signalscorresponding to M (M≧1) carrier frequencies of a RRU are multiplexedaccording to the I/Q data frame format shown in FIG. 7, and thentransmitted via GFP-T. Specifically, the I/Q baseband signals of carrierfrequency #m (m=1, 2, . . . M) from respective antennas are sequentiallyarrange firstly at the same sampling time, wherein the sequence of thesampling values of the I/Q baseband signals from respective antennas isthe same with the spatial location sequence of antenna array or antennagroup, the sampling values of the I/Q baseband signals from the sameantenna are sequentially arranged according to quadrature componentsampling values and In-phase component sampling values. Thus, at certainsampling time, the I/Q data length of the carrier frequency is 2WD bits;then M I/Q data with the same sampling time from M carrier frequenciesare sequentially arranged to form a data block with length being 2MWDbits. At last, L such data blocks are sequentially arranged according tosampling time increment to form an I/Q data frame, so the total bitlength is 2 MWDL. For the same reason, L is the minimal value whichmakes M WDL/4 to be integral. For example, the antenna number D of acertain RRU is 2, sampling bit width W is 11, and the number of thecarrier frequencies M is 3, then L=2, and the length of the I/Q dataframe is 33 bytes.

In the RRU-MU interface protocol structure shown in FIG. 4, thetransmission of I/Q data frame and control plane signaling frame is, forexample, shown in FIG. 8A. At the transmitting end, I/Q data is formedinto I/Q data frame via an I/Q data frame adaptation layer, then the I/Qdata frame is processed by a transmission scheduling unit and is mappedto 64B/65B code block by a 64B/65B coding unit, to form a GFP-T clientdata frame. The 64B/65B code block includes two kinds of controlcharacters, that is padding character and control plane signaling framecharacter, as well as the data character of the I/Q data frame. Thetransmission scheduling unit is responsible for the transmissionscheduling of the I/Q data frame and the control plane signaling frame.Specifically, the control plane signaling frame character enters intothe 64B/65B coding unit according to the following scheduling method:when the I/Q data frame character stream input buffer is null, if thecontrol plane signaling frame input buffer is not null, the controlplane signaling frame enters into the 64B/65B coding unit as controlcharacter, otherwise it is padded by the padding character. Then theGFP-T frame is formed and then is formed into VC (Virtual Container)/ODU(Optical Channel Data Unit) by a VC/ODU Mapping/concatenating unit, andis further formed into a STM-N/OTM-n frame, to realize the transmissionbased on SDH/OTN.

The processes at the receiving end are inverse processes of thetransmitting end, as shown in FIG. 8B. Firstly, the corresponding VC/ODUis separated from the STM-N/OTM-n frame and then payload thereof isextracted, the 64B/65B code blocks are obtained after being processed bya GFP-T frame processing unit, then the I/Q data frame character streamand the control plane signaling frame character stream are separated bya 64B/65B decoding unit. Wherein the I/Q data frame character stream isfurther processed by an I/Q data frame processing unit to obtain the I/Qdata streams of respective carrier frequencies from respective antennas,and the control plane signaling frame character stream is furtherprocessed according to the protocols of various layers of the controlplane as shown in FIG. 4.

For the two different RRU-MU interface modes described above, thedifference is in that the modes adopted in multiplexing the I/Q dataframes on the GFP-T frame are different. With regard to the first RRU-MUinterface mode, at the transmitting end the corresponding I/Q dataframes are formed for various carrier frequencies, and then transmittedby various super blocks, while at the receiving end various carrierfrequencies are distinguished according to various super blocks, at thesame time, the I/Q data frame character streams separated by the 64B/65Bdecoding unit are processed by the I/Q data frame processing unit toobtain the I/Q data streams of the respective antennas of thecorresponding carrier frequencies. With regard to the second RRU-MUinterface mode, at the transmitting end, the I/Q data of the respectiveantennas of different carrier frequencies are together formed into I/Qdata frame, while at the receiving end the I/Q data frame characterstreams separated by the 64B/65B decoding unit are processed by the I/Qdata frame processing unit to obtain the I/Q data streams of respectiveantennas of various carrier frequencies.

As discussed previously, in GFP code, only the low 4 bits of the controlcharacter in the 64B/65B code block are used to transmit control code,whereas the high 4 bits are used as the following control characterindication and the position identifier of the control code in theoriginal 8B/10B code stream. Since the present invention does not use8B/10B coding, the control character has only one usage to transmit thecontrol plane signaling frame besides being the padding character,therefore it is necessary to redefine the bits of the control character.As a nonrestrictive example for illustrating, FIG. 9 shows a kind ofdefinition for the control character, wherein the definition of the mostsignificant bit b7 is the same with that in the original GFP code, i.e.indicating whether the following bytes in the 64B/65B code block is thecontrol character, b6 is used to indicate if the control character isthe padding character, b5 is used as wireless frame synchronizationindication (which will be described in detail hereinafter), b4 isreserved for the future protocol extension, and the low 4 bits are usedto transmit the control plane signaling frame character stream. Whereinwhen b6 indicates that the control character is padding character, thelow 4 bits can be of any values, and will be omitted as paddingcharacter at the receiving end.

According to the present invention, GFP client management frame can alsobe used to inspect and maintain the GFP transmission link of the RRU-MUinterface. The usage of the GFP client management frame complies withthe GFP standard of ITU-T/ANSI. In the present GFP standard, the GFPclient management frame carries two kinds of client managementinformation, namely client signal failure (lose client signal) andclient signal failure (lose client character synchronization). Theclient management information is used to inform thetransmitting/receiving end in time to perform re-synchronizing torestore the normal link communication in the case that there are serioustransmission errors or GFP frame asynchronization (GFP frame delineationfailure) in the client signals.

In addition, in the BTS system using RRH technology, RRU management andcontrol information has at least three kinds of information: RRU-MUinterface link control, management and maintenance signaling, includingcontrol signaling such as link set-up, modification, and deletion,operation mode negotiation, rate negotiation, I/Q data frame formatnegotiation, etc; the parameter setup, on-off control, status inspectionand alarm of respective RRU radio models; and RRU operation andmaintenance information, for example, software/firmware upgrade,configuration management, etc. According to the present invention,except the operations with strict timing requirement such as on-offcontrol of RRU radio model (which will be described in detailhereinafter), other management and control information described aboveis within the control plane of the RRU-MU interface protocol shown inFIG. 4. The control plane signaling is carried on UDP/IP and/or TCP/IP.A typical example of the control plane signaling carried on UDP/IP isSNMP (Simple Network Management Protocol) message, and a typical examplecarried on TCP/IP is HTTP (HyperText Transfer Protocol), Telnet, andother control signaling described above. As discussed previously, thecontrol plane signaling is transmitted on SDH/OTN via GFP-T controlcharacter channel, and at the receiving end, even though the controlplane signaling stream can be extracted from the GFP-T control characterchannel, the GFP-T control character channel itself can't provide linklayer functions including packet delineation, reliable transmission, sothe above IP packet carrying the control plane signaling will further becarried on PPP and formed into frame by HDLC.

2. Wireless Frame Synchronization and RTT Estimation

As discussed previously, the present invention defines a bit for thewireless frame synchronization indication in the definition of thecontrol character, the bit defined can be used to synchronize thewireless frames and measure round trip transmission time delay (RTT),the principle thereof is shown in FIG. 10.

At first, the wireless frame timing at MU side is timing reference forall RRUs linked with the MU. In down stream, downstream I/Q data streamis send to the RRU via GFP-T, when wireless frame starting time at MUside appears, if the padding character is transmitted now, the wirelessframe synchronization indication bit (b5 in FIG. 9) of this character isset to “1”, if the I/Q data frame character is transmitted now, insert apadding character at this time immediately and set the wireless framesynchronization indication bit of this character to “1”. The time thatthe RRU receives the control character with its wireless framesynchronization indication bit set to “1” will be the wireless framestarting time at RRU side. There is certain delay between the wirelessframe timing at RRU side and that at MU side because of transmissiontime delay.

In up stream, upstream I/Q data stream is also send to the MU via GFP-T,once RRU has received the wireless frame synchronization indication ofdownstream, if the padding character is transmitted now, the wirelessframe synchronization indication bit (b5 in FIG. 9) of this character isset to “1”, if the I/Q data frame character is transmitted now, insert apadding character at this time immediately and set the wireless framesynchronization indication bit of this character to “1”. Similarly,there is certain delay between the wireless frame synchronizationindication received by the MU from the RRU and the wireless frame timingat RRU side because of transmission time delay. In this way, round triptransmission time delay (RTT) estimation can be obtained by calculatingthe difference between the wireless frame starting time fed back by theRRU and the starting time of the original wireless frame timing in theMU.

The object for the RRU to obtain wireless frame timing is that, sincethe RRU radio models (such as radio power amplifier, frequencysynthesizing unit, etc) often need period control signal with stricttiming requirement based on wireless frame timing, to perform theoperations concerning on-off control and mode transition of radiomodels, and transmitting/receiving switch in TDD (Time Division Duplex)system, etc, the RRU can generate the above signals periodically andlocally by using the wireless frame timing obtained (the starting andstopping time of various control signals can be configured and modifiedas parameters by above control plane signaling).

In addition, since there is certain delay jitter in transmission, theremay be certain jitter in the wireless frame synchronization indicationreceived by each RRU frame, therefore the periodicity of the wirelessframe timing can be used to perform Smooth Processing, to achieveaccurate wireless frame timing at RRU side.

3. RRU Time Delay Adjustment

As for wireless interface technology in TDD mode, it is necessary toensure that the upstream receiving and downstream transmitting performedby respective RRUs should be synchronized. Because the asynchronizationof transmitting/receiving timing of upstream and downstream in variouscells of TDD system will cause interference of transmitting/receivingtime slot in various cells, which will influence the advanced timingadjustment in cell handover, therefore, the synchronization ofrespective RRU wireless frame timing must be ensured. The discussionabout synchronization between various cells in TDD system may refer to3GPP (3rd-Generation Partnership Project) “TR25.836, NodeBSynchronization for TDD”. With regard to the wireless interfacetechnology in FDD (Frequency Division Duplex), RRU time delay adjustmentmay not be performed if strict wireless frame synchronization betweenvarious cells is not required, and may be performed if required.

Because of different transmission time delays, the wireless frame timingobtained by respective RRUs from MU by using the above method isdifferent, therefore for the TDD system, RRU time delay adjustment isrequired to ensure wireless frame timing synchronized between variouscells in CBTS. To this end, according to the present invention, the MUcan send the wireless frame timing to respective RRUs relatively earlieror later by using the obtained RRT measurements of respective RRU tomake the wireless frame timing of respective RRUs to be the same, so asto continuously track the RRT variation of respective RRUs and to keepthe wireless frame timing synchronized among respective RRUs.

FIG. 11 schematically shows the time delay adjustment based on RTTmeasurement. In FIG. 11, the topside graph is reference timing, themiddle is the timing of a certain RRU before time delay adjustment, andthe underside is the timing of the RRU after time delay adjustment, thearrow on the left points at the frame starting time corresponding torespective RRUs at MU side, the arrow in the middle points at the framestarting time received at RRU side, and the arrow on the right points atthe frame starting time fed back to the MU by the RRU, the round triptransmission time delay of reference timing is RTT₀, the round triptransmission time delay of the RRU is RTT₁. In FIG. 11A, RTT₁ of the RRUis longer than RTT₀ of the reference timing, that is, ΔRTT=RTT₁-RTT₂>0,therefore the MU has to advance the frame starting time sent to the RRUby ½ΔRTT, to ensure that the frame starting time receive at RRU side isaligned with the reference timing. The case of FIG. 11B is contrary tothat of FIG. 11A, RTT₁ of the RRU is shorter than RTT₀ of the referencetiming, that is, ΔRTT=RTT₁-RTT₂>0, therefore the MU has to delay theframe starting time sent to the ½ΔRTT, to ensure that the frame startingtime received at RRU side is aligned with the reference timing.

In fact the time delay adjustment above assumes that the transmissiontime delay in up stream is the same with that in down stream, which isadapted to most applications, but if the transmission time delay in upstream is different with that in down stream in specific application, anadjustment factor may be added to the advanced/delay value of wirelessframe timing sent to respective RRUs, that is, being corrected to½(1+γ)ΔRTT, where −1<γ<1, the adjustment factor can be determined byempirical value measured. In addition, the reference timing in the abovetime delay adjustment is a reference for the time delay adjustments ofall RRUs, whereas value selection of the reference timing will notinfluence relative timing synchronization among respective RRUs, butaverage frame starting time corresponding to respective RRUs at MU side,therefore the above reference timing is determined by average framestarting time corresponding to respective RRUs at MU side.

4. Sampling Clock Recovery and Frequency Synchronization

In the mobile communication system, the frequency stability of the BTSradio frequency unit is required to be relatively high, generally withan accuracy of 0.05 ppm, therefore the RRU requires the frequencyreference with high stability. Meanwhile, since the I/Q data stream istransmitted to the receiving end via asynchronous GFP-T channel, inorder to re-build I/Q data stream at the receiving end, it is necessaryto recover or obtain the bit synchronization clock of the I/Q datastream synchronous with that of the transmitting end.

In the BTS system using RRH technology, the MU can always obtain thefrequency reference with high stability, whereas the RRU has to recoveror obtain synchronous clock with high stability. On the one hand, toprovide the required frequency reference to radio frequency part, and onthe other hand, to re-build the downstream digital wireless signal datastream and generate the upstream wireless signal data stream by usingit. Therefore, two different methods can be used to get digital wirelesssignal data stream bit timing and the frequency reference with highstability required by the RRU. One method is adopting global commonclock, and a typical implementation is that both the MU and respectiveRRUs get frequency reference with high stability from GPS (GlobalPositioning System), and then use this frequency reference as thesampling clock source of the digital wireless signal data stream.Another method is adopting adaptive clock recovery which technology usethe feature that the continuous data stream transmitted itself hasconstant bit rate to recover the clock for the constant data streamthrough Phase Locked Loop (PLL), as shown in FIG. 12.

In FIG. 12, the MU uses the reference clock source with high stability,besides providing timing for the MU itself, in down stream, thisreference clock source provides timing for the GFP-T framing and VC/ODUmapping model, etc, provided at the transmitting end of the MU-RRUinterface at MU side, and the receiving end at RRU side recovers thedigital wireless signal data stream clock using the PLL based on FIFO(First In First Out), meanwhile the clock is also the referencefrequency source of the RRU. In up stream, the GFP-T framing and VC/ODUmapping model, etc, provided at the transmitting end of the MU-RRUinterface at RRU side send data using the synchronization clock obtainedin down stream, and the receiving end at MU side provides timing for theGFP-T frame decoding and VC/ODU demapping model, etc, using the abovereference clock source in the MU. In addition, the clock for theSTM-N/OTM-n interface at the transmitting/receiving end is directlyextracted from SDH/OTN line, instead of using the above client dataclock.

The interface method between RRU and CBTS of the present invention aredisclosed above in conjunction with the accompanying figures, but thedisclosures are intended not to limit the invention. Those skilled inthe art will recognize that various modifications and improvements maybe made to the invention according to the principle of the invention,without departing from the scope of the appended claims of theinvention.

1. A method for transmitting signal between a central channel processingMain Unit (MU) and one or more Remote Radio Units (RRUs) in CentralizedBase Transceiver Station system using remote radio head (RRH)technology, wherein the transmission channel between the central channelprocessing Main Unit (MU) and the one or more RRUs uses SynchronousDigital Hierarchy (SDH)/Optical Transmission Network (OTN), the methodcomprising: dividing the communication interface between the MU and theRRUs into a user plane and a control plane, the user plane mainly forcarrying I/Q sampling data concerning the user data, and the controlplane mainly for carrying the data concerning control signaling; formingthe I/Q sampling data concerning the user data carried by the user planeinto an I/Q data frame via an I/Q data frame adaptation layer, thenforming the I/Q data frame into a GFP frame via Generic FramingProcedure (GFP) and transmitting it on SDH/OTN; and carrying the controlsignaling of the control plane on User Data Protocol (UDP)/InternetProtocol (IP) and/or Transmission Control Protocol (TCP)/InternetProtocol (IP), and further carrying IP packet on Point to Point Protocol(PPP) and forming it into a frame by High-Level Data Link Control(HDLC), transmitting the HDLC frame including the control planesignaling on the SDH/OTN via control character channel of the GFP frame;further including the step of transmitting control plane signaling frameusing the control character channel of a GFP-T client data frame,comprising: at the transmitting end: forming the I/Q sampling data intoan I/Q data frame via the I/Q data frame adaptation layer; processingthe I/Q data frame by a transmission scheduling unit and then mapping itinto a 64B/65B code block by a 64B/65B coding unit so as to form a GFP-Tclient data frame, wherein the 64B/65B code block includes a paddingcharacter and the control plane signaling frame character, as well asthe data character of the I/Q data frame; forming the formed GFP-T frameinto VC (Virtual Container)/ODU (Optical Channel Data Unit) by a VC/ODUMapping/concatenating unit, then further forming a STM-N/OTM-n frame;and at the receiving end: separating the corresponding VC/ODU from theSTM-N/OTM-n frame and extracting its payload; obtaining the 64B/65B codeblocks after being processed by a GFP-T frame processing unit,separating the I/Q data frame character stream and the control planesignaling frame character stream by a 64B/65B decoding unit, wherein theI/Q data frame character stream is further processed by an I/Q dataframe processing unit to obtain the respective I/Q data streams ofrespective carrier frequencies from respective antennas.
 2. The methodfor transmitting signal of claim 1, further configuring thecommunication interface between the RRU and the MU, so that the RRU-MUinterface can support the transmission of a plurality of carrierfrequency wireless signals, and support the transmission ofmulti-antenna signals corresponding to the same carrier frequency andensure that the transmission time delays of respective antenna signalsare the same during transmission in the wireless BTS system adoptingmulti-antenna technology.
 3. The method for transmitting signal of claim1, wherein the step of forming the I/Q data frame into a GFP frame viaGFP at the interface between the RRU and the MU includes: transmittingthe wireless signals corresponding to M carrier frequencies of each RRUrespectively by respective super blocks of the GFP-T frame via timedivision multiplexing the transmission bandwidth of the GFP-T, whereM≧1.
 4. The method for transmitting signal of claim 1, wherein the stepof forming the I/Q sampling data concerning the user data carried by theuser plane into an I/Q data frame via an I/Q data frame adaptation layerincludes: sequentially arranging the I/Q baseband signals of the mthcarrier frequency from respective antennas at the same sampling time,where m=1, 2, . . . M, wherein setting the sequence of the samplingvalues of the I/Q baseband signals from respective antennas to be thesame with the spatial location sequence of antenna array or antennagroup, and sequentially arranging the sampling values of the I/Qbaseband signals from the same antenna according to quadrature componentsampling values and In-phase component sampling values to form a singleI/Q data frame.
 5. The method for transmitting signal of claim 1,wherein the step of forming the I/Q sampling data concerning the userdata carried by the user plane into an I/Q data frame via an I/Q dataframe adaptation layer includes: sequentially arranging the I/Q basebandsignals of the mth carrier frequency from respective antennas at thesame sampling time, where m=1, 2, . . . M, wherein setting the sequenceof the sampling values of the I/Q baseband signals from respectiveantennas to be the same with the spatial location sequence of antennaarray or antenna group, and sequentially arranging the sampling valuesof the I/Q baseband signals from the same antenna according toquadrature component sampling values and In-phase component samplingvalues; sequentially arranging M I/Q data with the same sampling timingfrom M carrier frequencies so as to form multiple data blocks; andsequentially arranging the multiple data blocks according to samplingtime increment so as to form a single I/Q data frame.
 6. The method fortransmitting signal of claim 1, wherein the definition of the controlcharacter in the code block is: the most significant bit b7 indicatingwhether the following bytes in the 64B/65B code block is the controlcharacter; the following b6 indicating whether the control character isa padding character; the following b5 indicating wireless framesynchronization; the last one in the high 4 bits, b4 being reserved forextension; and the low 4 bits for transmitting the in-band controlcharacter stream.
 7. The method for transmitting signal of claim 1,wherein GFP client management frame is used to inspect and maintain theGFP transmission link of the RRU-MU interface, the GFP client managementframe complying with the GFP standard of ITU-T/ANSI.
 8. The method fortransmitting signal of claim 1, further including the step ofsynchronizing the wireless frames and measuring round trip transmissiontime delay (RTT) using a wireless frame synchronization indication bit,the step comprising: making the wireless frame timing at MU side to betiming reference for all RRUs linked therewith; in down stream, sendingdownstream I/Q data stream to the RRU via GFP-T, when wireless framestaffing time at MU side appears, if the padding character istransmitted now, the wireless frame synchronization indication bit ofthis character is set to “1”, if the I/Q data frame character istransmitted now, insert a padding character at this time immediately andset the wireless frame synchronization indication bit of this characterto “1”; the time that the RRU receives the control character with itswireless frame synchronization indication bit set to “1” will be thewireless frame starting time at RRU side, a certain delay between thewireless frame timing at RRU side and that at MU side being appearedbecause of transmission time delay; in up stream, sending upstream I/Qdata stream to the MU via GFP-T, once the RRU has received the wirelessframe synchronization indication of downstream, if the padding characteris transmitted now, the wireless frame synchronization indication bit ofthis character is set to “1”, if the I/Q data frame character istransmitted now, insert a padding character at this time immediately andset the wireless frame synchronization indication bit of this characterto “1”; a certain delay between the wireless frame synchronizationindication received by the MU from the RRU and the wireless frame timingat RRU side being appeared because of transmission time delay; obtainingthe round trip transmission time delay (RTT) estimation by calculatingthe difference between the wireless frame starting time fed back by theRRU and the starting time of the original wireless frame timing in theMU.
 9. The method for transmitting signal of claim 8, further includingthe step of adjusting RRU time delay to ensure the wireless frame timingsynchronized among respective cells in CBTS, the step of adjusting RRUtime delay comprising: the MU sending the wireless frame timing torespective RRUs relatively earlier or later by using the obtained RRTmeasurements of respective RRUs, to make the wireless frame timing ofrespective RRUs to be the same, so as to continuously track the RRTvariations of respective RRUs and keep the wireless frame timingsynchronized among respective RRUs.
 10. The method for transmittingsignal of claim 1, further including: the RRU obtaining frequencyreference with high stability; and recovering or obtaining the bitsynchronization clock of the I/Q data stream synchronous with that ofthe transmitting end when re-building I/Q data stream at the receivingend.
 11. The method for transmitting signal of claim 10, furtherincluding: both the MU and respective RRUs getting frequency referencewith high stability from Global Positioning System (GPS), and using thisfrequency reference as the sampling clock source of the digital wirelesssignal data stream, so as to get digital wireless signal data stream bittiming and the frequency reference with high stability required by theRRU.
 12. The method for transmitting signal of claim 10, furtherincluding: using the feature that the continuous data stream transmitteditself has constant bit rate to recover clock for the constant datastream through Phase Locked Loop.
 13. The method for transmitting signalof claim 12, further including: in down stream: the MU using thereference clock source with high stability to provide timing for theGFP-T framing and VC/ODU mapping model at the transmitting end of theMU-RRU interface at MU side, the receiving end at RRU side recoveringthe digital wireless signal data stream clock using the PLL based onFirst In First Out (FIFO), at the same time the clock is also thereference frequency source of the RRU; and in up stream: the GFP-Tframing and VC/ODU mapping model at the transmitting end of the MU-RRUinterface at RRU side sending data using the synchronization clockobtained in down stream, the receiving end at MU side providing timingfor the GFP-T frame decoding and VC/ODU demapping model, using the abovereference clock source in the MU; the clock of the STM-N/OTM-n interfaceat the transmitting/receiving end being directly extracted from SDH/OTNline, instead of using the above client data clock.